6 Analysis Criteria

6.1 Stress assessment

6.1.1 General Stress assessment of the fine mesh analysis is to be carried out for the FE load combinations specified in Ch 4, Sec 8.

6.1.2 Reference stress

Reference stress is von Mises stress, σ_vm, which is to be calculated based on the membrane normal and shear stresses of the shell element evaluated at the element centroid. The stresses are to be evaluated at the mid plane of the element.

6.1.3 Permissible stress

The maximum permissible stresses are based on the mesh size of 50 x 50 mm as specified in [4.1] to [4.4]. Where a smaller mesh size is used, an area weighted von Mises stress calculated over an area equal to the specified mesh size may be used to compare with the permissible stresses. The averaging is to be based only on elements with their entire boundary located within the desired area. The average stress is to be calculated based on stresses at element centroid; stress values obtained by interpolation and/or extrapolation are not to be used. Stress averaging is not to be carried across structural discontinuities and abutting structure.

6.2 Acceptance criteria

6.2.1 Verification of stress results against the acceptance criteria is to be carried out in accordance with [6.1]. The structural assessment is to demonstrate that the stress complies with the following criteria:

λ_f ≤ λ_fperm

where:

λ_f : Fine mesh yield utilisation factor.

for shell elements in general
for rod or beam elements in general

σ_vm : Von Mises stress, in N/mm².

σ_axial : Axial stress in rod element, in N/mm².

λ_fperm : Permissible fine mesh utilisation factor, taken as:

Element not adjacent to weld:
- λ_fperm = 1.70 f_f for S+D
- λ_fperm = 1.36 f_f for S
Element adjacent to weld:
- λ_fperm = 1.50 f_f for S+D
- λ_fperm = 1.20 f_f for S

f_f : Fatigue factor, taken as:

f_f = 1.0 in general,
f_f = 1.2 for details assessed by very fine mesh analysis complying with the fatigue assessment criteria given in Ch 9, Sec 2.

Note 1: The maximum permissible stresses are based on the mesh size of 50 x 50 mm. Where a smaller mesh size is used, an average von Mises stress calculated in accordance with [6.1] over an area equal to the specified mesh size may be used to compare with the permissible stresses.

Note 2: Average von Mises stress is to be calculated based on weighted average against element areas:

where:
σ_vm-av is the average von Mises stress.

Note 3: Stress averaging is not to be carried across structural discontinuities and abutting structure.

6.2.2 Lower stool not fitted to a transverse or longitudinal corrugated bulkhead

Where a lower stool is not fitted to a transverse or longitudinal corrugated bulkhead, the permissible stresses given in [6.2.1] are to be reduced by 10% for the areas under investigation by fine mesh analysis.